Automatic Microdetermination of Carbon and Hydrogen RALPH 0. CLARK AND GORDON H. STILLSON Gulf Research & Development Company, P i t t s b u r g h 30, Pa.
In a series of experiments combustion oyoles for the analysis of a r a t h e r wide range of c o m p o u n d types were determined, using a s e m i a u t o m a t i c combustion unit. The unit is described and the conclusions d r a w n f r o m its operation are applied to fully a u t o m a t i c units now in routine Use. The combustion cycles employed are compared w i t h those of o t h e r automatic units previously dererihed.
I
N THE microdetermination of carbon and hydrogen in organio
Two automatic combustion units for the microdetermination of carbon and hydrogen have bmn described in the literature (3, 6 ) . Although in each case it was indicated that satisfactory analyses could be obtained, no extensive experimental data were given uuon which to base a fair comuarison of automat,icand manual methods from the standpoint of accuracy and precision. Because of the lack of such data,, and because of the wide variety of samples to be analyzed in this Laboratory, it seemed advisable not to construct a fully automatic apparatus at the outset, hut t o with what might he called ,tsemiautomtttic82 this type of unit the movable vaporization furnace is advanced by means of a motor-driven rotating screw, the rate
compounds, a combustion unit which operates automatically shows important advantages over the conventional, manudly operated apparatus. Stated briefly, the more outstanding advantams are: ~
l. One analyst can operate two units simultaneously; if only one unit is in operation, he is free to attend to other duties. 2. Combustion conditions can be held essentially constant, to give extremely uniform analyses. 3. Since the human factor has been minimized. inexperienced or untrained personnel can perform better analyses than are possible using a manually operated outfit.
oped, and oomhustion cycles adaptable to the rapid analysis of samples covering a wide range of compositions were determined. The fully automatic units now being used in this lahoratory are based, both in Construction and operat,ion, upon the semiautomatic units operated in routine analysis over a period of four years. Except for the addition of a mechanism for setting automatically the various phnses of the cycles, the automatic apparatus is similar in construction to the semiautomatic and has heen found capable of.the accuracy and precision indicated by the semiautomatic data. These data. IIPP r mn r~ t d anrl riiamaae.rl. The - ~ conchisions ~ . . . -.___ drawn from them apply without reservation t o the fully automatic units now in routine use. ~~
Figure 1.
S e m i a u t o m a t i c Carbon and Hydrogen 1J n i t
~~~
DESCRIFTION OF SEMIAUTOMATIC APPARATUS
The semiautomatic combustion unit is shown in Figure 1.
with the exception of the motor-driven vaporization furnace and
midified Abrahameeik absorption tubes ( 2 ) . the apparatus does
used in most laboratories for this type of analysis. Since the movable vaporization furnace is of prime importance in this discussion, it is described in some detail. The other components of the unit are not particularly unique in design or operation and have no direct bearing upon the data pres The vaporiaation furnace (Figure 2) is advanced t threaded, brass half-hearing which engages a. drivin:, Drlyl of Monel metal; both bearing and smew carry a 3/~inohthread, 12 threads to an inch. This design permits the furnace to he moved m a n u d y to any desired-position by merely lifting the bearing from the driving SCIRW, which is driven by a 100 r.p.m. unidirectional universd-type motor mounted below the Transite top of the unit, and connected to a shaft contained in one of the s u p ports. Power is transmitted from shaft to screw by a 30 t o 1 worm-and-gear combination, so that the driving screw rotates at 3.3 r.p.m. when the motor is operating at its maximum speed. By means of a rheostat it is possible t o obtain rates of furnace advance varying from 0.5 to 7 mm. per minute; the motor circuit is provided with B voltmeter calibrated in terms of rates of advance. When the vaporization furnace reaohes the combustion llllyI
F i g u r e 2.
Vaporization Furnace
423
424
V O L U M E 19, NO. 6
Table I. Routine Analyses by Semiautomatic >lethod, Showing hlean Error from Theoretical Values Compound
Carbon (C = 12.01) Found Calcd. Mean Error
70 2-Ethylhexyl salicylate N-Phenyl-N’allylthiourea bis(Benzy1mercapto)dibenzylmethane n-Butyl-pcresol Dibenzyly; phenetidine 1,1,3-Trimethylcyclopentane Octahydroanthracene Salicylicacid
70
71.76 71.67 71.67 62.45 62.46 62,48 79.14 79.04 79.00 80.46 80.43 80.46 83.27 83.24 83.26 85.76 85.62 85.76
Parts/1000a +0.6
-0.5 -0.4
fO
4
-0,4
+1 6
Hydrogen ( H = 1.008) Found Calcd. Mean Error
% 8.92 8.94 6.46 6.48 6.50 6.52
%
Parts/1000”
8.81
+14
6.29
+29
6.40
+19
10.03 9.82 9.87 7.33 7.30 i.36 14.51 14.37 14.45
+13
90.43 90.41 +0.3 9.73 9.59 90.44 9.63 60.81 60.87 +O,l 4.57 4.38 60.90 4.41 Tributyl 53.46 53.70 -2.6 10.19 10.12 phosphate 53.63 10.06 Average mean error - 1 . 1 to + O . 6 fl Calculated from mean of two analyses reported,
+8
-11
+
9
+25
-
1
+ 16
furnace it is stopped automatically as the threaded bearing runs o f fonto a smooth section of the driving screw. A vaporization furnace must not only be capable of reaching operating temperature in an extremely short time, but also must have the property of cooling rapidly after a determination, so that the next sample can be passed through it without danger of premature vaporization. The furnace described is 6.35 cm. long over-all and contains six heating elements, each consisting of a length of hlundum thermocouple tubing (4.25-mm. outside diameter) upon which is wound sufficient No. 30 annealed platinum wire to give the furnace a total resistance of 5.5 ohms with the elements connected in series. The windings are spaced so that the end of the furnace toward the combustion furnace operates at a temperature approximately 50” C. higher than the other end. The elements are held in place by thin Alfrax disks at each end of the furnace. An Alfrax tube (3.4-cm. outside diameter 5.25 em long) surrounds the elements and fits into circular grooves in the end disks. The space between the Alfrax tube and the outei brass furnace shell is filled with loosely-packed asbestos fibers. The temperature is adjusted through a variable transformer and is measured by means of a thermocouple with flexible leads. The temperatures of the vaporization furnace, combustion furnace, preburner, and heating mortar are all read, through manipulation of a selector switch, on a single millivoltmeter calibrated directly in degrees Centigrade. PROCEDURE FOR SEMIAUTOMATIC COMBUSTION
For the sake of brevity, in the description of procedure which follows, only those samples which may be weighed in a conventional weighing boat are considered. The vaporization furnace is placed 5 cm. from the combustion furnace, and the sample is located in the combustion tube so that half the boat is outside the movable furnace on the side toward the combustion furnace. After the absorption tubes have been opened to the flow of oxygen, the heat isturned on in the vaporization furnace, with the current adjusted to give a maximum temperature of about 500” C. The furnace is advanced at 0.75 mm. per minute for 4 minutes, after which it is set in motion at the maximum rate of 7 mm. per minute. This rate is maintained until the sample has reached a position in the tube where it begins to vaporize, a t which time the furnace is slowed down to 0.75 mm. per minute. (The distance between the two furnaces at this stage is dependent entirely upon the burning characteristics of the sample. This point is illustrated under Evaluation of Combustion Cycles.) After the sample b s vaporized into the filled section of the combustion tube, the driving motor again is operated at maximum speed and the temperature of the furnace is raised to 650” C. Three minutes after the furnace disengages from the driving screw, its heating current is turned off and it is returned to its starting position manually.
.Is mentioned previously, one end of the vaporization furnace is designed to give a higher temperature than the other. Kormally this additional heat is adequate to burn any carbonaceous deposit which may have formed on the walls of the combustion
tube at the entrance to the combustion furnace. By allowing the silver wire plug of the combustion tube filling to extend about 6 mm. beyond the end of the combustion furnace, even the most difficultly combustible deposits have been found to burn completely without resorting to longer heating periods. Reburning of t.he combustion tube following vaporization of the sample is unnecesary, provided the sample is not distilled out of its container in less than 4 minutes. Counterflom of the sample is mused almost invariably by distilling too rapidly. EXPERIMENTAL DATA
If we concede t,o a new analytical procedure certain advantages such as economy of time, effort, or materials, the accepted standards of accuracy and precision still must be met before a final appraisal is possible. “Accuracy” in the present instance may be defined as a measure of the correctness of analytical results, based upon the calculated carbon and hydrogen cont,ent,of a compound of known composition. The accuracy of semiautomatic combustions probably can be judged best by comparing the mean deviations or [‘mean errors” ( 5 ) from the true or theoretical values, with the mean deviations found in comparable manual combustion results. Rather than examine a large number of data from both methods, routine analyses of a small number of samples representing a wide range of structures and compositions are compared. Because the data were taken from routine determinations, it. is not possible tmo give both manual and semiautomatic analyses for the same samples. Table I lists analyses, with errors, for nine compounds analyzed with the semiautomatic unit, while comparable figures for nine other samples analyzed manually are shown in Table 11. Errors and deviations are expressed in “parts per thousand” rather than percentages based upon composition, in order that they may be independent of the magnitude of carbon and hydrogen content. The term ‘‘precision” is used to describe the consistency of results among themselves, and is an index of their reproducibility. A comprehensive statistical study of the accuracy and precision of t,he microdetermination of carbon and hydrogen was made by the late Francis W. Power, S. J . ( 5 ) . I n order that the semiautomatic results may be compared wit’h manual dat’a reported by Power, precision is expressed in terms of “average deviation” and “standard deviation.” (The authors believe it is permissible to assume that a high proport,ion of the analyses reported in the survey conducted by Power were performed manually.) Average deviation, A D , and standard deviation, SI),are calculated by means of the formulas
Table 11. Routine Analyses by Manual Method, Showing Mean Error from Theoretical Values Hydrogen (H = 1 . 0 0 8 ) Carbon (C = 12.01) Found Calcd. Mean Error Found Calcd. Mean Error % % Parts/1000a % 5% Parts/1000a $24 Aiiisole 77.88 77.74 $2.3 7.63 7.45 77.96 7.62 $27 +0.9 7.70 7.51 9-Methyl-6-tert- 8 0 . 6 1 8 0 . 5 6 butyl-phenyl 8 0 . 6 4 7.72 benzoate -2.6 4 54 4.38 +32 p-Hydroxy60.80 60.87 4.50 benzoic acid 6 0 . 6 2 8.59 8.25 +0.6 4-36 4-3Ietboxy-1- 7 8 . 6 4 7 8 . 6 5 methylben78 77 8.51 zene Butylbenzene- 6 6 . 0 8 6 6 . 0 8 -0.6 8.72 8.72 1 sulfon-n66.00 8.75 butylamide 0.0 7.05 6.97 $13 Dibenzyl-p83.09 83.13 anisidine 83.16 7.06 1.6 14.66 14.37 18 a-Hexadecylene 8 5 . 8 3 8 5 . 6 2 85 69 14.59 9.82 - 4 +l.Z 9.77 p-terf-.lmyl80.44 80.43 phenol 80.61 9.78 -0.4 10.09 9.93 +16 2.4-Dibutyl75.71 75.73 catechol 75.69 10.os Average mean error - 1 . 2 to 1 . 3 +21 Calculated from mean of two analyses reported. Ciirupound
+
+
+
+
J U N E 1947 3
..
e
.’
I
425 TOTAL TRAVEL: 7 6 M M TOTAL TIME: 30 MrN
ROYER C Y C L E
..
n 25MM.
7
w 4 - 3_) 0
25MH.
52 Z I
I
TOTAL TRAVEL= 6 3 M M .
1
3
~ TOTAL ~ TIME: 2 6 M I N
deviation is somen-hat less than for semiautomatic determinations. I t is suspected that Power’s hydrogen results have been corrected by means of blank water determinations, since n-ith Pregl absorption t u b e such close agreement with the calculated hydrogen content usually cannot be expected even under better-than-average conditions. The qalicylic acid analyses, performed with closed-type absorption tubes, have not been correrted. EVALUATION OF COMBUSTION CYCLES
H A L L E T T CYCLE
The preceding discussion shows that the semiautomatic combustion procedure is capable of results at least equivalent to those obtained by manual methods. I t is believed that, in addition, it is applicable to a greater variety of sample types than are the automatic procedures developed by Hallett (5) and Royer (6). The differences between their automatic cycles and the cycles used in the semiautomatic procedure just described are illustrated I . . . 1 in Figure 3. 0 5 IO 15 20 25 38 The Royer cycle uses a constant rate of TIME- MINUTES furnace advance throughout the combustion Figure 3. Comparison of Royer, Hallett, and Semiautomatic Cycles period. While the over-all rate can be changed to a certain extent by adjustment of the motor governor. no adjustment within the cycle is poadl+d,+da+ .......... .dn sible. A constait advance rate of 2.5 mm. per minute is recAD = N ommended, 1.5 times faster than Hallett’s lowest rate of advance and 3.3 times faster than the lowest rate used in the semiautomatic cycle. This would seem to impose a definite SD = d d ? dz d’, Ai’ . . . . , d:, limitation on the Royer apparatus as regards its application t o compounds which must be vaporized under carefully controlled where d represents the difference between the measured value in conditions. question and the arithmetical mean of all the measured valucs in Hallett’s cycle should have n ider application than Royer’s a series of ATvalues. because the lower initial rate of furnace advance should allow the A comparison of semiautomatic results with data from Power’s higher boiling samples to distill out of the boat into the tube a t a investigation and from earlier manual determinations made in uniform rate, thus minimizing the danger of counterflow of vapor this laboratory is shon-n in Table 111. through too rapid vaporization. However, the Hallett cycle Another type of comparison can be made from the data tabuassumes that all compounds being analyzed will have been vaporlated in Table IV. Shoxm here are a large number of determinations on salicylic acid by the semiautomatic method, alongside a large number of manual analyies of benzoic acid reported by Power ( 5 ) . The actual error, average deviaticn, and standard Table 111. Comparison of Deviations on AIany Analyses deviation are shown for each series. Standard Deviation, In selecting data for Tables I11 and I\-, certain “wild” results Parts per 1000 No. of Analyses Carbon Hydrogen were rejected according to the folloiviny empirical rule i(4): 8
,
.
+ + + . . . ..
The deviation of the doubtful result from the arithmetic mean divided by the average deviation; if the quotient exceeds 2.5 the doubtful result is rejected. The suspected figure is not included in the computation of the average deviation.
Manual Semiautomatic Power
76
9.1
1.4 1.4 2.5
54
218
10
18
IS
Table IV. Comparison of Semiautomatic Determinations on Salicylic Acid with Power’s Results for Benzoic Acid
DISCUSSION OF DATA
That the accuracy obtained with the semiautomatic unit is at least equal to that of manual analyses is shown in Table I and 11, summarized in Table V. While this conclusion is based on the limited number of analyses reported, it is borne out by all the analyses performed in this laboratory over a period of years, using bcth methods of combustion. Based upon a comparison with manual results, as shown in Tables I11 and IV, the precision attainable with the semiautomatic apparatus appears to be entirely satisfactory. In Table IV, which compares a large number of analyses of two conventional compounds by both methods, it will be seen that for the semiautomatic carbon analyses the deviation from theoretical of the mean determined values is less than for manual combustion as reported by Power. In the case of the hydrogen analyses Power’F:
No. of analyses Theoretical, % Mean of determined values. % Average deviation of a single analysis from mean, parts per 1000 Standard deviation of a single analysis f r o m mean, parts per 1000
Carbon Hydrogen Salicylic Salicylic acid Bensoic acid Benzoic (semiautoacid (semiautoacid matic) (Power) matic) (Power) 120 34 120 34 60.87 68.84 4.38 4.96 60.85 68.99 4.53 5.02 1.3 1.8
,
2.0
22
18
2.6
29
24
Table V. Comparisonof Mean Errors in Manual and Semiautomatic Methods Manual meth,od Semiautomatic method
Mean Error, Parts per 1000 Carbon Hydrogen -1 2 to + 1 . 3 - 1 . l t o $0.8
x
V O L U M E 19, NO. 6
426 iaed into the combustion zone by the time the speed of the furnace is increased to a maximum. Experience gained during the development of the semiautomatic procedure indicates that in many cases this advance a t maximum speed could occur before all of the sample has distilled into the filled portion of the combustion tube; or, in extreme cases, before any of the sample has entered the combustion zone. Either Occurrence o m bring about erroneous results (8). In the semiautomatic procedure the first period af rapid furnace advance (represented by the left,hand "hump" in the diagram, Figure 3) has no influence upon the vaporization time of the sample; its sole purpose is to save time in moving the liquid or liquefied sample d o a n the combustion tube. It is only after the furnace resumes its minimum rate of advance that vaporization begins. Experiment has shown that a i t h cycles 4 or 5 most highboiling materials are vaporized campletely into the combustion zone before the second period of maximum furnace advance uccurs (represented by the right-hand "hump" on Figure 3). This second maximum advance period is used only for reducing thc time necemary to bring the movable furnace up to the eomhustion furnace for the burning-off of any carbonaceous material. Three minutes me allowed far this latter operation. The five semiautomatic cycles shown in Figure 3 will give accurate results on materials ranging from liquids boiling a t about 100" C. to materials boiling up t o a t least 220" C. a t l mm. [bis(2-hydroxy-3tertbutyl-5-methy1phenyl)methanel. The analyst, by e q e r i ence, chooses the cycle according to physical data and indicated structure given for the sample. If such information is not available, cycle 3 is tried first. Observation during this first determination usually tells whether cycle 3 is giving satisfactory results or, if i t is not, indicates a more apprapr determinations.
Table VI. Comparison of Results Obtained Using SemiD.~ .... .->" allett Combustion Procedures ~
- - - ~ ~ - - % .
.rbon
Per Cent Hydrogen Semi-
autoe t t Cdod. matio 85 60.89 66
....I _.,_
phenol Beneohteof 81.53
64 82.46 51
81.94 81.83 81.61 8 1 . 7 5 81.12 81.31 81.53 81.56 8 1 . 7 1
Royer Hallett Calod.
4.52 4.48 4.57 4.38 4.47 4.66 4.56 11.52 11.54 11.52 11.52 11.42 11.61 11.49 9.03 9.05
9.10 8.95 8.93 9.06 8.55
8.88 8.76 8.90 8.76 8.89
8.93
The opinions expressed regarding the relative merits of the Hallet, Royer, and semiautomatic procedures are supported rather well by an experiment in which three compounds of known composition were analyzed consecutively by the three cycles, using the semiautomatic unit. The results of this experiment are shown in Table VI. These data indicate that the three procedures are equivalent when applied to a. compound which burns easily, such as salicylic acid. All three still give comparable results when applied to a compound such as 2,4,6triterebutylphenol which is moderately resistant to combustion, Hourever, when the comparison is extended to a cornpound which burns with difficulty and requires preselected m d controlled combustion conditions, only the semiautomatic method gives consistent and x c u m t e results. Such a ease is represented by the alkylphenylhenzoate shown in Table VI. I n this determination it wak observed that the rate of furnaee advance recommended by Royer was much too fast for eontrolled vaporization of the sample; and that, when the HaIlett cycle was used, a major portion of the sample still remained as a.
Firure 4.
liqi vanmu
Fully Automatic Carbon and Hydrogen IJnit
L/V~ U L Lq
m u
CONVERSION TO AUTOMATIC OPERATION
The semiautomatic unit was converted to fully automatic operation by the installation of a system of microswitches actuated by motor-driven cams, so designed that the advance rate and temperature of the vaporiaation furnace could he set automatictically for all phases of a particular combustion cycle. In the automatic unit the five semiautomatic cycles shown in Figure 3 can be selected by means of a 5-gang, 10-circuit selector switch. Thus the automatic apparatus, like tho semiautomatic, covers %widerange of compound types. I n eases which require special consideration, the unit may he set far semiautomatic operation and controlled according to the judgment of the analyst. If in fully automatic operation i t is observed that the appropriate cycle has not been chosen for it particular sample, usually it is possible to rectify the error through proper manipulation of the selector mitch. The automatic unit now being used for routine operation is shown in Figure 4. Incorporated into the automatic unit, as well as into the earlier semiautomatic apparatus, is a timing device which turns on the current to the heating mortar, combustion furnace, and prehurner one hour before the operator arrives in the morning. With these heaters at approximately operating temperatures, an analysis may be started without delay. This feature makes possible one extra determination per day. ACKNOWLEDGMENT
The authors wish to thank Gladys Bistline and Eileen Harry for their capable performance of many of the analyses used in this discussion. LITERATURE CITED
(1) Clark, R. O., addStillson, G. H., IND.END. CHEM 494 (1940). (2) Ciark, R. O.,and Stillson. G.H., Ibid., 17, 520 (19 131 L. T.. Ibid.. 10. 101 (1938). , ~ Hdlett. , (4) Xolthoff, I. M., a i d Snndeil. E. B., "Textbook Inorganic Analysis." p. 276, New York, Maomillan Co., 1943. (5) Power, F . W., IND.END. CHBM.,AKAL.ED.,11, 660 (1939). (6) Royer, G. L.. Norton, 9.R., and Syndberg. 0. E., Ihid., 12, 688
__
_II
(1940).
P ~ E S E N Tin ED part before the Division of Analytiesl and Mioro Chemistry. Symposimm on Microdetermination of Carbon snd Hydrogen, a t the 108th' Meeting of the AMERICAN C x m n c ~ rSOCIETY, . New York. N. Y .
